Death is sad, no getting round that.
Avoiding uncomfortable emotional experiences is not a useful life skill.
Learning how to navigate sadness, frustration, regret, anger, distress etc etc constructively is an incredibly useful life skill.
My kids respond well to this sort of approach...
gently discuss how all life ends eventually, perhaps by reference to a book you've read, or something that comes up on telly, or you can get books from the library that are written to cover this topic, similar to books that help prepare children for a new sibling or going into hospital etc.
Talk about how loving something/someone well includes difficult choices sometimes and putting their needs ahead of your own.
Talk about how you all love the dog and don't want dog to suffer, but illness happens to people and animals and sometimes they get better and sometimes they don't and their life ends with that illness.
Talk about how to honour the great love you feel you would want to be brave and help make sure her death was not traumatic or painful for her but that you comfort her and give her an easy passing of preventing her death is not possible.
Explain how courage might be needed and that is the greatest gift you can give her and it's how you can show love in this sad situation.
Reassure, he can sort her and give her cuddles through it if he wants, or if he prefers you will do that for him. Reassurevhim it won't be a sudden may surprise and say that one of the silver linings of anticipating losing something you love is that you get to choose how to say goodbye.
Ask if he would like to join you in reading her a special poem about what she has meant to you all. Maybe her favourite walk of she's still up to it, it something nice to smell if she's not, perhaps something that smells of him if he doesn't want to be there when she goes but so he knows she could smell him and how that would have been a reassuring thing for her because he represents safety and love.
Talk about how tears are powerful and it's ok to shed tears because grief is just love with nowhere to go and it means dog was loved.
Talk about how grief passes with time and it's natural that it hurts for a while and that's what funerals are for, they help give your grief somewhere to go and mark this important loss.
Then later maybe make a picture album together or put a picture on the wall of her.